A screen printing method that provides great cost advantages is typically used for forming the electrodes in bulk solar cells in which semiconductor crystal substrates are used. In the screen printing method, an electrode paste consisting of, for example, silver particles, a resin, glass frit, a solvent, and the like, is used. With the screen printing method, the electrode paste is applied to a printing mask that is formed with a predetermined pattern, and the electrode paste is transferred and thus printed onto a print substrate (semiconductor substrate) through the printing mask by moving a printing squeegee over the printing mask. Then, the electrode paste printed on the semiconductor substrate is fired at a predetermined temperature that is in accordance with the materials in the electrode paste, whereby an electrode having a desired pattern is obtained.
When an electrode of a solar cell is formed, it is necessary to reduce the electrode-area percentage of the area of the semiconductor substrate on the light receiving surface side in order to capture more solar light on the light receiving surface. Furthermore, it is necessary for forming an electrode having a low resistance to increase the cross-sectional area of the electrode. Thus, when an electrode of a solar cell is formed, it is necessary to form an electrode that has a small electrode width, a large electrode height, and a high aspect ratio.
One of the methods for obtaining an electrode having a high aspect ratio by using a screen printing method is to form a multi-layer electrode by printing an electrode paste a plurality of times. With this method, an electrode paste that is to be the first layer is first printed on the substrate and is then fired or dried at a predetermined temperature. Thereafter, an electrode paste that is to be the second layer is printed in a superposed manner on the electrode paste of the first layer and is then fired or dried again at a predetermined temperature. Superposition printing is then repeated until the desired electrode height is obtained, thereby forming a multi-layer electrode.
Moreover, there is a selective emitter structure as a solar cell structure in which an electrode portion is formed by using superposition printing. With this structure, in order to increase the photoelectric conversion efficiency of the solar cell, a highly doped layer (low-resistance diffusion layer, hereinafter, sometimes referred to as a terrace) is formed in a region larger than the electrode on the light receiving surface side of the semiconductor substrate and thus the sheet resistance is reduced, thereby increasing the conductivity. Moreover, a low doped layer (high-resistance diffusion layer) is formed in the region other than the terrace on the light receiving surface side of the semiconductor substrate, thereby inhibiting the recombination of electrons. When the selective emitter structure is used, a light-receiving-surface-side electrode is formed by printing an electrode paste for forming the light-receiving-surface-side electrode on the low-resistance diffusion layer in a superposed manner.
Typically, when superposition printing of an electrode paste is performed, an alignment mark having a specific shape is used. For example, when an electrode paste is printed twice in a superposed manner, the shape data and positional data on the alignment mark of the second layer are registered in advance as a reference image in an image printing apparatus. Then, at the same time as the printed material (electrode paste) of the first layer is printed on the surface of the semiconductor substrate, an alignment mark that has the same shape as the alignment mark described above is printed on the surface of the semiconductor substrate.
Next, when an electrode paste of the second layer is printed, the printing stage is first finely adjusted so that the positional data on the alignment mark of the second layer stored in advance in the image printing apparatus matches the positional data on the alignment mark that has the same shape and is printed together with the electrode paste of the first layer, and then the electrode paste of the second layer is printed. At this point in time, the print position of the electrode paste of the second layer to be superposed on the electrode paste of the first layer is aligned with reference to the positioning reference position that is determined in accordance with the position of the alignment mark. This operation is repeated a given number of times to form an electrode portion. By repeating this operation a given number of times for superposing an electrode paste, an electrode is formed.
When an electrode is formed by performing such superposition printing, if the electrode paste portion (upper-layer electrode paste portion) to be printed next protrudes from the low-resistance diffusion layer (terrace) or the electrode paste portion (lower-layer electrode paste portion) that is printed first (printing misalignment), the photoelectric conversion efficiency of the solar cell is reduced. In other words, if the light-receiving-surface-side electrode protrudes from the low-resistance diffusion layer (terrace) and overlaps with the high-resistance diffusion layer, the contact resistance between the light-receiving-surface-side electrode and the substrate increases and thus the properties of the solar cell are reduced. As a result, the photoelectric conversion efficiency of the solar cell is reduced. Moreover, if the upper-layer electrode paste portion protrudes from the lower-layer electrode paste portion, the light receiving area is reduced. As a result, the photoelectric conversion efficiency of the solar cell is reduced. Thus, high superposition printing accuracy is required between the lower-layer electrode paste portion and the upper-layer electrode paste portion. Therefore, it is important to reduce any error that affects the high superposition printing accuracy.
In practice, however, it is not possible to eliminate all errors in superposition printing accuracy. Thus, it is also important to address errors that actually occur by providing a margin such that the superposition itself does not fail.
There are various factors that cause an error in superposition printing accuracy, such as a design error and a manufacturing error. However, an error in superposition printing accuracy has a tendency to have a correlation with a factor that is a positional relation with respect to a specific point, e.g., a tendency to have a correlation with the distance from the printing reference point that is used when printing is performed. Such factors include extension and a rotation error of a printing mask because of the repeated use of the printing mask. All of the errors increase or decrease depending on the distance from the reference point that is used as a reference when print positioning is performed.
The former error occurs because some of the elastic deformation of the screen remain, i.e., becomes irreversible, when the printing mask is repeatedly used, and the deformation rate per unit length essentially has a correlation with the distance from the reference point. The latter error is an error in the angle in the rotation direction that the whole pattern of the superposed electrode pastes may have, and this error is proportional to the angular error that occurs and to the distance from the reference point to each point. These errors are typically small at a point close to the reference point and large at a point away from the reference point. Because the errors have such characteristics, there is a risk of dramatically increasing the errors depending on the location; therefore, it is important to take appropriate measures to deal with these errors compared with other kinds of error factors.
In view of such a problem, for example, Patent Literature 1 proposes a method of reducing extension and distortion of a printing mask. In Patent Literature 1, the percentage of the area of a screen mesh made of a rigid material, such as a metal, in the whole area of the screen mesh is set to 40% or lower in the combination printing mask having a screen mesh made of a synthetic resin and the screen mesh made of a rigid material, thereby reducing extension, distortion, and the like of the printing mask that occurs as the number of times printing is performed increases. The purpose of this method is to eliminate the errors themselves.